1. Field of the Invention
The invention concerns force measuring apparatus of the optical sensor type enabling limitation of eccentricity defects, more particularly measuring apparatus in which the optical sensor is optical fiber based.
It also, but not exclusively, concerns the application of measuring apparatus of this kind to weighing apparatus of the bathroom scales type.
2. Description of the Prior Art
It is well known that when a force of the same magnitude (or the same weight) is applied to different points on a sensor the amplitude of the measured signal can vary from one point to another. This is known as an "eccentricity" defect. The invention concerns apparatus intended to limit variations in the measured signal due to this unwanted effect.
Many types of weighing apparatus use strain gauges glued to test bodies. This design has the drawback of requiring sensitivity adjustment because the signal obtained is sensitive to the point of application of the measured force.
To overcome these drawbacks it has been proposed to replace the strain gauges by an optical sensor, in particular an optical fiber based optical sensor.
There are three main families of optical fiber sensors: intensity sensors, interferometer sensors and polarimeter sensors.
A polarimeter sensor is based on a monomode optical fiber, for example a standard silica fiber that can have a very low intrinsic birefringence (a so-called "LoBi" or "Low Birefringence" optical fiber, or a more conventional fiber with a very low residual birefringence). Linearly polarized light is injected into the optical fiber. The light source used for this can be a laser, for example a laser diode. However, the emitted light does not have any particular polarization direction. Accordingly, it must be linearly polarized using a polarizer before it is injected into the optical fiber.
A stress is applied to the optical fiber, a pressure force (weight) in the context of the invention. This causes birefringence which modifies the state of polarization. The core and the cladding of the optical fiber assume an elliptical shape, this being the phenomenon inducing the birefringence.
The force F exerted on the optical fiber (the weight placed on the weighing platform) is deduced from the phase shift .psi. due to the birefringence using the following equation: ##EQU1## where .lambda..sub.vac is the wavelength of the light wave in vacuum and K is a constant such that: ##EQU2## where:
______________________________________ n core material refractive index n = 1.458 P.sub.12 photo-elastic matrix coefficient P.sub.12 = 0.27 P.sub.11 photo-elastic matrix coefficient P.sub.11 = 0.121 .nu. Poisson's coefficient .nu. = 0.17 E Young's modulus of the silica E = 7 10.sup.10 N/m.sup.2 2r fiber diameter 2r = 125 .mu.m ______________________________________
Equation (1) clearly shows that the phase shift .psi. is a direct function of the force F (weight) applied to the optical fiber and, additionally, that it is independent of the length L of the stressed optical fiber.
To measure the phase shift at the output of the optical fiber, the luminous intensity is in practise measured using an opto-electronic converter. This is generally a photodiode that detects the aforementioned luminous intensity and converts it into an output electrical signal. The luminous intensity I obeys the following law: EQU I=I.sub.0 .multidot.(1+cos (.DELTA..psi.)) (3)
where I.sub.0 is the intensity without any phase shift, i.e. in the absence of any stress.
If a quarter-wavelength optical plate and a polarizer imposing an orientation .theta. are placed between the light source and the optical fiber, the intensity obtained obeys the following law: EQU I=I.sub.0 .multidot.(1+cos (.DELTA..psi.-2.theta.)) (4)
Measuring apparatus of this type is described in French patent application No. 95 06853 filed Jun. 9, 1995.
FIG. 1 is a diagrammatic representation of its general structure.
The measuring apparatus 1 includes a base 3 on top of which is a platform 2 to which a force F is applied in a direction perpendicular to the plane of the base 3 and the platform 2. An optical fiber 6 is disposed between these two members, advantageously coiled upon itself to form one or more turns. The radius of curvature of the turns must naturally be compatible with the mechanical characteristics of the optical fiber.
In the example shown, the measuring apparatus 1 includes a light source D.sub.e, for example a semiconductor laser diode. Between a first end or input face 60 of the optical fiber 6 and the diode D.sub.e is a first polarizer P.sub.01. In a similar way, a second polarizer or analyzer P.sub.02 is placed between the second end 61 of the optical fiber 6 and an optoelectronic converter D.sub.1, for example a photodiode. The latter converts the luminous intensity received into an output electrical signal V.sub.S. The polarization axes are parallel to each other and preferably at 45.degree. to the direction of application of the force F. Of course, it is generally necessary to provide various supplementary optical devices, especially focusing devices, which are well known in themselves.
The electrical output signal V.sub.S is then processed by conventional electronic circuits, either in analog form or, preferably, in digital form. All that is required for this is to provide an analog-digital converter and control circuits for a digital display unit such as a liquid crystal display. A microprocessor could equally well be used, executing various preprogrammed functions.
The force measuring apparatus briefly described above has many advantages, including:
integration of the effect along the optical fiber 6, i.e. measurement of the total phase shift; PA1 little influence of centering, which depends only on the plate-fiber linking conditions; PA1 extreme thinness, the thickness specific to the optical element (the optical fiber 6) being virtually negligible (typically 1/10 millimeter); PA1 an equally small displacement; PA1 high sensitivity: typically 20 g for a load of 140 kg and an optical fiber length of 1.5 m; and PA1 many construction options since the load is distributed over all of a ring of optical fibers.
In a preferred embodiment circular or even annular structures are used, i.e. structures having an open central area.
It has nevertheless been found that, contrary to what is suggested by the theory, the eccentricity defects are significant, typically in the order of .+-.1%.
Accordingly, an object of the invention is to limit the unwanted effects of eccentricity defects whilst retaining the advantages of apparatus of the type just mentioned.